Patent GB-640031-A (GB)
Title: Electric measuring circuit
IPC: G01R17/04
Filed: 1947-07-29
Abstract: 640,031. Voltage measurements. BENDIX AVIATION CORPORATION. July 29, 1947, No. 20438. Convention date, Aug. 24, 1946. [Class 37] [Also in Group XXXVIII] In apparatus for measuring a voltage by balancing against a search source of periodically varying voltage the search source is connected to the source of voltage to be measured when the search voltage is less than the voltage to be measured and to a source of higher voltage when the search voltage is greater than this higher voltage, indicating means responsive to the currents flowing in the connections so established being provided. In the humidity-measuring apparatus shown, a hair element 38 as it changes in length with changes in humidity moves a tap 36 over a potentiometer resistance 34 and thus varies the voltage applied to the anode 44 of a diode 46 through a resistance 48. The cathode 52 of the diode 46 is connected through the secondary 54 of a transformer 56 to the moving contact arm 60 of a potentiometer resistance 58 which is connected between earth and a tap 18 at a potential of 50 volts on a potentialdividing resistance 14 across a 150 volt battery 12. The primary 62 of the transformer 56 is energized from an A.C. source 64 to impress on the cathode 52 a corresponding alternating voltage. The search voltage potentiometer 58 has an auxiliary contact 70 connected to the high voltage terminal of the battery 12. The arm 60 is driven continuously clockwise by a motor 72 so that it traverses the potentiometer resistance 58 and then momentarily engages the contact 70. A character-bearing disc 74 is mounted on the shaft of the motor 72 and a printed strip 76 is periodically pressed against it by a striker 78 moved by a relay 82. During the period when the arm 60 is out of engagement with the resistance 58 or the contact 70 it is maintained at the potential of the 50-volt tap 18 through a stabilizing resistance 71. As the arm 60 rotates it impresses on the cathode 52 a search potential starting at 50 volts positive and decreasing to zero. At a point where the voltage at the cathode 52 becomes nearly equal to that at the anode 44 the alternating potential superimposed on the cathode 52 periodically drives it negative with respect to the anode 44. A corresponding pulsating voltage appears across the resistor 48 and is amplified by a valve 90 and applied to the control grid 118 of a gasfilled trip valve 120. This starts to discharge and produces a positive pulse in its cathode circuit which initiates a discharge in gasfilled relay control valve 144. A reservoir capacitor 158 is thus discharged through a relay 156 which energizes the relay 82 so that the tapper bar 78 drives the printing strip 76 into engagement with the printer wheel 74 causing a figure to be printed indicative of the angular position of the arm 60 at the time when the search voltage was equal to the voltage to be measured. Continuing rotation of the arm 60 decreases the D.C. component at 52 still further, thus increasing the signal input to the grid 118 of the trip valve 120. The control grid is alternately driven positive and negative, a diode 124 limiting the negative signal excursion and preventing interruption of the discharge in the valve 120. Continuing rotation disengages the arm 60 from the resistance 58, but its potential is brought to 50 volts by current flow through the resistor 71. This ensures that A.C. signal transmission through the diode 46 is prevented and also maintains the anode 173 of a diode 174 negative with respect to its cathode 172, preventing operation of the reset circuit. With further movement the arm 60 engages the reset contact 70, raising its potential to 150 volts, and swinging the anode 173 positive with respect to the cathode 172 so as to develop across the resistor 182 a 50 volt pulse triggering the reset valve 162. A discharge is established in this valve and extinguished in the trip valve 120. The circuit is thus prepared for a repetition of the measuring operation, which is initiated by engagement of the arm 60 with the potentiometer resistance 58 in its subsequent rotation. Variables other than humidity may be similarly measured by use of appropriate condition responsive elements driving variable resistors or by the use of resistors or other circuit elements themselves directly responsive to the condition being monitored. Specification 640,030, [Group XL (c)], is referred to.